Sounds like 'eth-oh-sux-ih-mide'

Ethosuximide is used to treat epilepsy. Find out how to take it safely and possible side effects. Ethosuximide is also called Zarontin.

What is ethosuximide?

Ethosuximide is used to treat epilepsy by preventing absence (petit mal) seizures. It is called an anti-seizure medication. Ethosuximide works by controlling the neurotransmitters (chemicals) in the brain. Read more about anti-seizure medication.

August 2022:Temporary change of brand of ethosuximide capsules (Zarontin capsules)  
For a few months from August 2022 until early 2023, Zarontin 250mg capsules will not be available in New Zealand. While Zarontin is not available, Ethosuximide Essential Generics is the substitute brand. This is made by the same manufacturer as Zarontin, but at a different  site.

Learn more Supply issue: Zarontin capsules


In New Zealand, ethosuximide is available as capsules and a liquid syrup.

  • The dose of ethosuximide will be different for different people, depending on your age and response to the medication or if you are taking other medicines.
  • Your doctor will start you on a low dose and increase your dose slowly over a few weeks.
  • Always take your ethosuximide exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much to take, how often to take it, and any special instructions.

How to take ethosuximide capsules

  • Timing: Ethosuximide is usually taken twice a day, usually about 10–12 hours part. Try to take it at the same time(s) each day, as this will help you to remember to take your doses regularly.
  • Food and drink: Swallow your capsules whole with a drink of water. You can take ethosuximide with or without food.
  • Missed dose: If you forget to take your dose, take it as soon as you remember if it is within 4 hours of when your dose was due. But if more than 4 hours have passed since when the dose was due, just take the next dose at the right time. Do not take extra doses to make up for a forgotten dose. If you are not sure what to do, ask your healthcare provider.
  • Keep taking ethosuximide every day:  See tips to help you remember to take your medicines regularly.  It may take a few weeks before you notice the full benefits of ethosuximide. Don't stop taking ethosuximide suddenly; talk to your doctor or nurse before stopping.

For information on how to give ethosuximide to children, see ethosuximide information for parents and carers. 

Things to consider while you are taking ethosuximide

  • Avoid drinking alcohol while you are taking ethosuximide. Alcohol can increase your chance of side effects such as drowsiness.
  • Tell all your healthcare providers that you are taking ethosuximide. This includes your doctors, nurses, pharmacists and dentists.
  • Avoid driving and doing other tasks or actions that need you to be alert until you see how this medicine affects you.
  • Ethosuximide can interact with other medicines. Tell your doctor or pharmacist about all medicines you are taking including over the counter medicines, herbal and complementary medicines or recreational drugs. 
  • Tell your doctor if you are pregnant, planning to become pregnant, or breastfeeding.

Possible side effects

Like all medicines, ethosuximide can cause side effects, although not everyone gets them. Often side effects go away once your body gets used to the new medicine.

Side effects What should I do?
  • Feeling sick (nausea)
  • Being sick (vomiting) 
  • Runny poo (diarrhoea)
  • Stomach ache
  • This is quite common when you start ethosuximide.
  • Tell your doctor if these are ongoing.
  • Feeling sleepy, drowsy or tired
  • Feeling dizzy or lightheaded 
  • This is quite common when you start ethosuximide.
  • Be careful when driving or using tools until you know how this medicine affects you.
  • These effects put you at risk of falls, and injuries especially if you are an older person. Tell your doctor if you are concerned.
  • Do not drink alcohol – it makes these effects worse.
  • Headache
  • Muscle weakness
  • Hiccups
  • These may happen when you first start taking ethosuximide.
  • Tell your doctor if these bother you.
  • Loss of coordination, trouble concentrating
  • Tell your doctor.
  • Skin rash, skin peeling or blisters
  • Stop taking ethosuximide and see your doctor immediately.
  • Mood changes, low mood, thoughts or talk of suicide and self-harm
  • Tell your doctor immediately or phone Healthline free on 0800 611 116.
  • Signs of problems with your blood cells, such as mouth ulcers, fever, chills, sore throat, unexplained bruising or bleeding
  • Ethosuximide can affect your blood cells – this is serious but very rare.
  • Tell your doctor immediately or phone Healthline free on 0800 611 116.
Did you know that you can report a side effect to a medicine to CARM (Centre for Adverse Reactions Monitoring)? Report a side effect to a product.

Learn more

The following links have more information on ethosuximide.

Ethosuximide New Zealand Formulary, Patient Information (adults)
Ethosuximide New Zealand Formulary, Patient Information (children)
Zarontin Medsafe Consumer Information, NZ


  1. Ethosuximide NZ Formulary, NZ
Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 02 Aug 2022